Sphingosine kinase 1 (SPHK1) is an oncogenic enzyme that is upregulated in a
wide range of human tumours and is associated with cancer progression. The product
of SPHK1's activity, sphingosine-1-phosphate (S1P) enhances metastatic potential by
promoting cancer cell migration and invasion. Our group has previously reported that
in vitro SPHK1 inhibition potentiates the effects of docetaxel, a key treatment of
prostate cancer (PCa) , implicating a potential therapeutic role of SPHK1.
The work presented in this thesis investigated the mechanism for docetaxel-induced
apoptosis in PC-3 cells and demonstrated the involvement of a SPHK1-dependent and
-independent pathway. The dose-dependent inhibition of SPHK1 by docetaxel led to
an initial loss of enzyme activity followed by a decrease in SPHK1 expression. Further,
in prostate cancer cell models, SPHK1 inhibition had a significant chemosensitising
potential leading to a 4-fold reduction in the effective dose of docetaxel.
To link SPHK1 upregulation in prostate cancer cells to potential targets, this thesis
investigated the nature of the interaction between SPHK1 and prosaposin signalling
pathways. Prosaposin is a neurotrophic secreted protein involved in cancer progression
and chemoresistance. Recent reports suggest that prosaposin activates SPHK1
in cancer cells. The work presented in this thesis demonstrates that prosaposin
knockout mice exhibited a significant reduction in SPHK1 activity in the prostate
and seminal vesicles. In prostate cancer cell lines, SPHK1 activity correlated with
the amount of secreted, but not intracellular prosaposin. This suggests a role of the
prosaposin/G-protein coupled receptor (GPCR) signalling. Blocking prosaposin in
prostate cancer cells induced a signi cant decrease in SPHK1 activity and expression.
Conversely, exogenous prosaptide TX14A, derived from the trophic sequence of saposin
C, enhanced SPHK1 activity and expression.
In turn, SPHK1 activation is essential for prosaposin secretion, but not for its
expression. Increased SPHK1 expression or exogenous S1P triggered prosaposin
secretion, while inhibition of SPHK1 using speci c siRNA or pharmacological inhibitor
drastically decreased prosaposin secretion. Both prosaposin and SPHK1 signalling
pathways were shown to be involved in the production of cytokines and angiogenic
factors such as plasminogen activator inhibitor-1 (PAI-1), macrophage migration
inhibitory factor (MIF) and pentraxin-3 (PTX3).
These findings were extended to the clinical situation, and circulating prosaposin levels
were determined in prostate cancer patients and compared with healthy controls.
Increased prosaposin serum levels correlated with tumour stage. The work in this
thesis has shown for the rst time the cross-regulation of prosaposin and SPHK1 in
prostate cancer. The mechanism and physiological relevance of this cross-regulation in
prostate cancer cells has been investigated and found to have significant therapeutic
and biomarker potential in advanced prostate cancer.